Power semiconductor elements are expected to save energy because they can reduce energy loss during power conversion. Performance improvement in power semiconductor elements have been hitherto achieved by using silicon (Si) semiconductors, but Si semiconductor elements appear to have reached a point of no further performance improvement owing to the limitation of physical properties of Si.
On the other hand, silicon carbide (SiC) has excellent physical properties, for example, an about 10 times higher electric field strength at dielectric breakdown, an about 3 times wider forbidden band, and an about 3 times larger thermal conductivity than those of Si, providing a prospect of further performance improvement in power semiconductor elements, and therefore SiC semiconductors using this material are demanded to spread rapidly.
SiC has many polytypes, and among them, the polytype of 4H is generally used for SiC substrates for SiC semiconductor elements which are expected to be next-generation semiconductor elements for power conversion. In addition, from the view point of using the step-controlled epitaxy, 4H-SiC substrates having an off-angle are regarded as main stream. Forming effectively an epitaxially grown layer on a SiC substrate having a low off-angle of less than 4°, particularly 1° or less is expected to be able to reduce the production cost of SiC semiconductor elements and to control the anisotropicity of properties of SiC semiconductor elements, leading to their effective utilization for various fields in society.
SiC substrates are usually given a predetermined off-angle from the (0001) Si plane or the (000-1) C plane when cut off from a SiC ingot. The cut SiC substrate is surface-processed by, for example, polishing, and then used in the form of an epitaxial wafer having an epitaxially grown layer formed on the surface of the substrate.
In this process, when the epitaxially grown layer is contaminated by a different polytype or contains many carrier-trapping centers, semiconductor elements to be formed on the layer will deteriorate their performance and reliability. In addition, when the epitaxially grown layer captures a large amount of impurity nitrogen, control of nitrogen concentration can be carried out only within the range higher than the concentration of the impurity nitrogen, which narrows the controllable range of nitrogen concentration and makes it difficult to fabricate high voltage semiconductor elements on the layer. In order to fabricate reliable high voltage semiconductor elements, it is indispensable to reduce the contamination of the epitaxially grown layer by different polytypes, the generation of carrier-trapping centers, and the capture of impurity nitrogen.
It is known that a lower off-angle of the SiC substrate tends to cause the contamination of the epitaxially grown layer by different polytypes, and a substrate having an off-angle of 4° is considered main stream as a substrate having an off-angle which can reduce the contamination by different polytypes. It is reported that a raw material gas having an low atom number ratio of carbon atoms to silicon atoms (C/Si ratio) is required to be introduced in order to reduce the contamination by different polytypes when an epitaxially grown layer is formed on a substrate having an off-angle of less than 4°, and particularly 1° or less (Patent document 1, and Non-patent document 1).
However, forming an epitaxially grown layer at a low C/Si ratio causes a problem of increase in carrier-trapping centers and the capture of impurity nitrogen which are due to carbon vacancies (Non-patent document 2, and Non-patent document 3).